Every version of Android has a hidden Easter egg that's always fun to find and the latest Android 12 is no different.
To find this new one, head to Settings, then scroll down and tap About Phone.
In there, find and tap on Android Version, which will take you to an information page about, unsurprisingly, your version of Android.
There should be a section at the top that says Android version 12. Tap this repeatedly and the screen will transform to show a clock, complete with the Material You colors based on whatever background you have set.
You'll need to tap and drag the minute hand on the clock and move them round until both hands point straight upward in the 12 o'clock position. You'll feel a small vibration pulse at each hour position, but it's only at the actual 12 position that the clock will disappear and you'll be greeted with the Easter egg screen: the number 12 in a sea of circles, all decked out in colors to match your background.
There's no further hidden game (as we've experience with older versions of Android) and it would maybe have been fun to at least be able to pop some of those circles or move them around a bit. Still, it's pretty enough to look at. To dismiss it, you can swipe up from the bottom of the screen the way you would to switch apps.